The present invention relates to high reflectance, narrow bandwidth, multilayer mirrors for reflecting radiation in the soft x-ray region, and more particularly, to mirrors for reflecting high intensity radiation.
As shown in FIG. 1, a typical mirror 15, designed to maximize reflectance and minimize bandwidth for the soft x-ray region, comprises alternating sublayers of greater optically absorptive material 20 and lesser optically absorptive material 30 on a substrate 40. Each bilayer pair 35 of such greater 20 and lesser 30 optically absorptive material typically has optical thickness of half of the desired wavelength. The multiple bilayers 35 thus provide an interference effect in order to provide high reflectance and a narrow bandwidth.
As shown in FIG. 2, mirror 15 generates a standing wave 60 in reflecting applied radiation 65 (FIG. 1) of the desired wavelength. The greater absorptive sublayers 20 typically contain the nodes 70 of that standing wave 60. As shown in FIG. 3, absorption is not symmetrical about the node 70 within the greater absorptive sublayers 20. Furthermore, the highest absorption in each greater absorptive sublayer 20 occurs at its boundaries, and the peak absorption among all the greater absorptive sublayers 20 occurs at the boundary between the topmost greater absorptive sublayer 20 and the ambient medium 80.
Such mirrors are susceptible to damage when the applied radiation 65 is of high intensity. The exact mechanism by which such high intensity radiation 65 causes damage is not known, but it is believed to involve strong electric fields, high absorption, or heating based on absorption. If the peak electric field squared or the peak absorption occurs at an interface of a greater absorptive sublayer 20 or near the ambient medium 80, any physical stress the peak electric field squared or the peak absorption causes, such as heat, is not readily dissipated and may damage the mirror 15.
Recent efforts to develop x-ray lasers have been hampered by such multilayer mirror damage. It would be highly desirable to provide double, triple or even multiple pass amplification in such lasers, but multilayers mirrors used in such experiments are usually destroyed even by a single exposure. The radiation intensity at the surface of such mirrors might be as high as 2-4*10.sup.8 Watts(W)/centimeter(cm).sup.2 which, for an exposure of 1*10.sup.-9 seconds(s), results in energy density of about 0.2-0.4 joules(J)/cm.sup.2. High reflectance, narrow bandwidth, multilayer damage resistant mirrors would also be useful for reflecting high intensity radiation in other applications besides x-ray lasers, such as free electron lasers, x-ray projection lithography, imaging and holography of biological specimens, and material studies.
U.S. Pat. No. 4,147,409 to Joseph H. Apfel discloses a multilayer laser reflector with reduced electric field intensity. Such a design is intended for use in the visible and near infrared region and would not provide a workable damage resistant, high reflectance, narrow bandwidth mirror in the soft x-ray region.